It is well known that thin film transistor-liquid crystal display (TFT-LCD) is one of the main products of current flat panel display. It has become an important display platform for modern information technology and video product. Since liquid crystal itself does not emit light, a backlight module is required in order to implement the display function thereof.
Nowadays, mainstream backlight modules generally use a side-access type backlight. In the backlight module, a light-emitting element is firstly provided on the inner side of a side wall of a backlight frame, and then light is guided to a display area through a light guide plate (LGP) mounted on the backlight frame. To ensure the light coupling efficiency between the light-emitting element and the light guide plate, i.e., the efficiency of incident light guided into the light guide plate from the light-emitting element, a reflective sheet is provided at the bottom of the light guide plate along the vertical direction, which is the direction along a vertical line passing through the light guide plate and the bottom of the backlight frame. Generally, the size of the reflective sheet along the horizontal direction, which is the direction orthogonal to the vertical direction and includes a longitudinal direction and a transversal direction, is equal to that of the light guide plate. That is to say, the reflective sheet is flush with the light guide plate.
In practice, it is found that when the reflective sheet extends along the horizontal direction to a lower location of the light-emitting element in the vertical direction, the efficiency of incident light guided into the light guide plate from the light-emitting element increases at least 3% compared with the situation when the reflective sheet is flush with the light guide plate. In some actual examples, the brightness of the display as measured increases from 4735 nit to 4890 nit. Therefore, the reflective sheet should be arranged as extending to the bottom of the light-emitting element along the vertical direction as much as possible.
At present, there are two types of typical methods for the arrangement of the reflective sheet. In one method, the reflective sheet is bonded to the bottom of the light guide plate, and extends to a lower location under the light-emitting element along the vertical direction. However, in this case, the expansion and compression of the reflective sheet induced by temperature and humidity to which the reflective sheet is subjected would be restricted, thereby resulting uneven bright and dark areas, i.e. wave-shape strips, present in the picture generated when such a backlight module is used. In the other method, the reflective sheet is placed between the light guide plate and the backlight frame in a free state, and extends to the lower location under the light-emitting element along the vertical direction. However, in this case, the reflective sheet mounted in the backlight module may be retracted from the lower location under the light-emitting element along the vertical direction, thereby failing to achieve the object of enhancing the efficiency of incident light.